Grenades for grenade launchers

I was watching the military channel, and I saw the M32 grenade launcher. I really want to build something like it with knex. But here's the problem: I have yet to find a grenade that can fit in some sort of barrel. I'd use Carlz's grenade, but the rubberbands on the outside of it would GREATLY decrease the range. We need to make a smooth grenade, any ideas?